1. Field of the Invention
The present invention relates to a semiconductor device for varying the mobility of electrons by light irradiation. The semiconductor of the present invention can be used as an optical gate transistor or the like which performs the switching of current with a photodetector or optical signal as its gate.
2. Related Background Art
Several types of photodetectors have been proposed.
A pn junction type photodiode is in use. This photodiode modulates optical current by changing the number of carriers such that the pn junction, inversely biased to less than the breakdown voltage, generates electron-hole pairs when receiving electromagnetic irradiation.
In recent years, a photodetector which uses the transition between the sub-bands of a quantum, well as an infrared photodetector has been proposed. FIG. 1A and FIG. 1B, for example, show illustrations of the energy bands of an infrared photodetector of the type proposed in Applied Physics Letter Vol. 50, No. 16, pp. 1092-1094. In this photodetector, an electric field is first applied vertically, i.e., in the horizontal direction in FIG. 1A, to the well face of a multiple quantum well structure 70 which has been processed by doping. If light equivalent to the transition energy between the sub-bands of the quantum well is irradiated onto this photodetector, the electrons which have made a transition from the ground state to the pumping state, as shown in FIG. 1B, penetrate the barrier layer by tunneling and become hot electrons 71 which flow between the well layers. As a result, it becomes possible to detect infrared light, i.e., the light which has photon energy equivalent to the transition energy between the sub-bands.
A velocity modulation transistor (VMT) has been proposed in Japanese Journal of Applied Physics Vol. 1, No. 6, pp. L381-L383. This transistor is capable of performing high-speed switching which exceeds the ultimate limit of velocity of field effect transistors, i.e., a switching faster than the transition time of electrons from the source to the drain. The transistor's structure is such that while maintaining the number of electrons N in the channel at a substantially constant level, the mobility of electrons flowing in the channel, i.e., the velocity, is varied by the voltage applied to the gate to modulate the current between the source and drain. As a result, it becomes possible to execute a high-speed operation independent of the time required to change the number of carriers N, i.e., the transition time of electrons between the source and drain.
However, in the conventional examples mentioned above, the problems set forth below have been encountered.
In the pn junction type photodiode, the response time depends on the RC time constant determined by the capacitance of the pn junction C and the load resistance R, making it difficult for this type of photodiode to respond at high speed.
In a photodetector of the type shown in FIG. 1A and FIG. 1B, a high electric field should be applied to the quantum well structure in advance to prepare hot electrons. Consequently, there exists a considerable amount of current generated by tunneling, the so-called dark current. Moreover, the response time of a detector element of this type is ultimately determined by the transition time during which the optically pumped carriers penetrate between the quantum well layers to reach the electrode. This transition time is estimated at approximately 30 ps. In this respect, although the response time of the VMT is not restricted by the transition time as described earlier, it is still impossible to make its response time sufficiently satisfactory because a voltage should be applied to the gate electrode to modulate the mobility of the electrons. Moreover, the response time of the element is restricted by the RC time constant determined by the gate capacitance C and the series resistance R of the applied voltage supply resistance and channel resistance.